Operation of internal combustion engines, particularly diesel engines, usually results in the generation of particulate matter (PM) including inorganic species (ash), sulfates, small organic species generally referred to as soluble organic fraction (SOF), and hydrocarbon particulates or “soot.” Various strategies have been used over the years for preventing release of PM into the environment. For some time, on-highway machines have been equipped with exhaust particulate traps as standard equipment. More recently, off-highway machines have been the subject of attention with regard to reducing/controlling PM emissions. While various designs for on-highway exhaust particulate filters have proven to be relatively effective in their intended environment, there are certain shortcomings to the designs if subjected to the demands placed on many off-highway machines. Moreover, even the most successful of the many known on-highway systems suffer from various drawbacks.
Conventional exhaust particulate filters are available in a wide variety of designs. Commonly, a fibrous material or porous ceramic material is positioned in the path of exhaust exiting an engine, and collects particulates to prevent then escape via the engine exhaust stream. The accumulation of PM within a filter tends to increase the resistance of the filter apparatus to the flow of exhaust gas, necessitating some means of cleaning the filter material, as reduced flow can affect fuel consumption, altitude capability, engine response and exhaust inlet and outlet temperatures. While filters may be mechanically cleaned, they tend to clog frequently enough that manual or mechanized cleaning is impractical.
Common approaches for removing accumulated PM from exhaust filters have been to regenerate the filter with heat or with heat and catalysts. Since typical engine exhaust temperatures are only intermittently, if ever, high enough to initiate combustion of accumulated PM, some means for periodically generating extra heat energy and/or the provision of combustion-facilitating catalyst materials has typically been used. Intake throttling and exhaust throttling are also used to increase the temperature of exhaust gas to a temperature sufficient to initiate combustion of the accumulated PM. In any case, once combustion of the accumulated PM is induced, it can be consumed rather than passed out to the environment, and returning the filter to a desired state. These approaches, however, have their own disadvantages.
On the one hand, catalyst materials tend to be relatively expensive, having obvious disadvantages where a manufacturer seeks to commercialize a particular filter design. On the other hand, applying additional heat energy, with or without catalysts, typically requires some type of auxiliary burner or other relatively complicated subsystem. Many filter regeneration strategies have also suffered from the inability to reliably initiate or maintain acceptable combustion of the accumulated PM without temporarily blocking or reducing exhaust flow through the portion of the filter to be regenerated. In some instances, for example where the filter design relies upon propagation of a flame against the flow of exhaust gases, the exhaust gases can apparently blow out the flame. In other instances, where a flame is propagated with the flow of exhaust gases, rather than unsustainable combustion the PM can burn out of control, raising the temperature of the filter and related components above temperatures they are designed to withstand.
Engineers have relied in many instances on relatively complicated valving or bypass systems to control exhaust throughput to a portion of the filter being regenerated. This is done in an attempt to achieve self-sustaining combustion of accumulated PM without combustion getting out of control. The moving parts in such systems not only add complexity, weight and expense to the associated machine, they also can fail, particularly when subjected to the rigors of off-highway work.
A still further problem associated with conventional regeneration strategies is the excessive energy consumed in the regeneration process. In the case of auxiliary burner strategies, extra fuel is typically injected into and combusted in the exhaust gas stream to raise the temperature of the exhaust gases to a regeneration temperature. Various electrically regenerated systems have been proposed, however, they too tend to require an excess of energy to regenerate. Conventional alternators and batteries commonly used with diesel engine-powered machines are not typically sufficiently powerful to regenerate an exhaust particulate filter, and supplementary electrical power such as a connection to a power grid or an oversized alternator is commonly required.
U.S. Pat. No. 5,293,742 to Gillingham et al. (“Gillingham”) is directed to a trap apparatus having tubular filter elements, for use in particular with diesel engines. In the design set forth in Gillingham, filter tubes surrounded with filter material such as yarn or various foams are used. The filter tubes are positioned within a housing, subdivided into different sectors. During regeneration, parts of the housing can be closed off and the filter tubes therein heated via electric heaters to effect regeneration. While the design of Gillingham may serve its intended purpose, it suffers from a variety of drawbacks. On the one hand, an elaborate system is necessary to direct exhaust gases to only certain parts of the filter apparatus, while restricting flow of exhaust gases to certain parts for regeneration. Restricting flow inherently reduces the efficacy of the filter and possibly the overall exhaust system, as regeneration is often necessary relatively frequently, often numerous times a day depending upon operating conditions. As is the case with many electrically regenerable filters, discussed above, Gillingham may also need a relatively large amount of electrical energy to successfully regenerate.